1. Field of Invention
The invention relates to electric power generation, and, more specifically, to large scale electric power energy storage and fast response frequency regulation. The electric grid is continuously attempting to balance the load (demand) with the power capacity (supply) connected to the grid. A perfect balance between supply and demand is impossible without the use of energy storage, load banks, flexible power sources and fast response supply systems. Grid operators manage these supply demand discrepancies by forecasting capacity requirements several days in advance, but, inevitably, there are errors in the forecast or gaps in the planned supply. Therefore, the grid requires fast response multi-megawatt energy storage and delivery systems that can react to the grid's requirements in real time. This has been an elusive requirement. The most successful type of system in use today is pumped storage, but it is costly and difficult to build and contains significant inertia, so its response is relatively slow and better suited for non-real time applications. High speed flywheels have been used for this purpose, but high costs and lack of multi-megawatt capabilities have significantly limited their adoption. The present invention addresses these challenges.
2. Description of Prior Art
Energy storage systems for electric power frequency regulation have increased in importance with the introduction of intermittent power sources, including wind and solar plants. In order to work properly and address the needs of the electric grid, these storage systems need to be capable of storing or delivering several megawatts of electric power for relative short periods of time on the order of 4 to 20 seconds, with a round trip efficiency (RTE, defined as the average ratio of power output versus power input over a period of ˜24 hours) greater than 70%, and capital costs in $/KW installed lower than the cost of adding new capacity, and a service life greater than twenty years. Because the currently-available and/or proposed energy storage systems fail to meet these requirements, inefficient and dated gas turbine power plants, or reciprocating engines known as “peakers”, have seen a sharp increase in demand in recent years. These peaker power plants are relatively compact and easily deployed, but, because of their inefficiency and their use of fossil fuels, they offset the renewable benefits of the intermittent solar and wind farms.
The currently available and/or energy storage devices for electric power generation include compressed gas systems, battery based systems, pumped storage systems and high speed flywheels. For example, Nakhamkin, (US Publication 2010/0251712) describes a compressed air energy storage system with a relatively low RTE of 65% and high capital costs to implement. Manz (US Publication 2011/0137481) describes a battery-based energy storage system for wind farms, but the upfront costs of this system remain high and its service life inadequate. Ley (U.S. Pat. No. 3,405,278, 1965) describes a pumped storage system for hydroelectric plants; such systems represent the bulk of the energy storage and frequency regulation systems in use today, but their use is limited due to high capital costs, difficulty in finding proper sites, and the extensive permitting required. Hockney (U.S. Pat. No. 6,614,132, 2003), Gray (US Publication 2010/0237629), Han (US Publication 2010/0231075) and Palmer (US Publication 2011/0120806) all describe high speed flywheels, with a combination of vacuum chambers and magnetic levitation to minimize friction, hollow shafts and flexible materials to minimize structural stresses at higher speeds, and multiple flywheel systems. However, these flywheel systems are either too costly or cannot provide multi-megawatt capacity to the grid. In fact, the Hockney patent was owned by Beacon Power, a flywheel energy storage company that recently filed for bankruptcy due to the inadequate economics of these systems.
The present invention provides large scale multi-megawatt real time frequency regulation with round trip efficiencies of approximately 80% and above by using gravitational and elastic potential energy along with rotational kinetic energy storage. The embodiments involve energy storage in the form of rotational kinetic energy and potential energy in the form of compression and tension in springs, along with gravitational potential energy of a mass. Several other inventions have been described in applications unrelated to large scale electric power energy storage and frequency regulation. For example, Easton (U.S. Pat. No. 373,061, 1887) and Hitt (U.S. Pat. No. 424,418, 1890) describe clock winding mechanisms using a torsional spring. Korfhage (U.S. Pat. No. 1,952,030, 1931) describes an automatic clock winding mechanism that stores potential energy in both a torsional spring and in the rise/fall of several masses. Dennis (U.S. Pat. No. 3,986,580, 1975) describes an energy storage device that can be used in mechanical transmissions, in which energy stored in several linear springs is arranged as an equivalent torsional spring. Yang (U.S. Pat. No. 5,269,197, 1993) describes an air-stream activated energy storage device, in which energy is stored in springs that are compressed through centrifugal forces acting on inertial masses. Fielder (U.S. Pat. No. 7,127,886, 2006) describes a self-winding electric generator that provides consistent power from wind and other intermittent energy sources by incorporating the use of a torsional spring similar to that used in self-winding clocks. While the above inventions incorporate the use of springs to store energy and, in the case of Yang, a centrifugal force that acts upon a spring, none of them are designed for use as a fast response multi-megawatt frequency regulation device for the electric grid, independent of the energy source.